Evolution of the Local Structure within Chromophoric Mn-O₅ Trigonal Bipyramids in YMn_{1- x}In _xO₃ with Composition

We have investigated the local environment around Mn³⁺ and In³⁺ ions in YMn_{1- x}In _xO₃ chromophores to understand the origin of the intense blue color for small values of x in these solid solutions. While X-ray diffraction results provide an average description of the trigonal bipyramidal (TBP) units about Mn/In atoms with five oxygens surrounding the cation, the X-ray absorption near edge structure (XANES) as well as extended X-ray absorption fine structure (EXAFS) of these materials clearly suggest the presence of two different TBP environments, one of which is similar to MnO₅ TBP in YMnO₃. EXAFS in conjunction with first-principles calculations show that replacing larger In³⁺ ions by smaller Mn³⁺ ones additionally gives rise to another TBP strongly distorted along the axial direction, expanding one of the axial Mn-O bonds by ∼11%. The relative fraction of these two environments changes in close agreement with the global stoichiometry with the elongated TBP, therefore, being dominant in the regime of the low Mn content. This local structural difference is responsible for the intense, but relatively narrow, absorption feature in the red-yellow region of the absorption spectrum, and hence YMn_{1- x}In _xO₃ appears blue for small Mn dopings. This distortion is relatively less abundant in Mn-rich compositions, and therefore, such compositions appear black, controlled by the wide absorption feature of the trigonal bipyramid coordination with Mn-O bond lengths that are essentially the same as those in YMnO₃, covering the entire visible range. The chromophore properties are, thus, governed by the ratio of these two MnO₅ TBP environments, one with a characteristic optical absorption giving it a blue color and the other absorbing over the entire visible range.